This application pertains to the field of potting compounds for electrical power apparatuses, especially to the field of potting compounds for electrical ballasts for fluorescent lights.
Various compounds are known for potting electrical circuits: that is, encasing an electrical circuit in a container within an impervious material to protect the circuit from the environment,and/or to improve the heat dissipation or electrical characteristics of the circuit, or, for electrical power circuits, for electrical safety.
Two major classes of compounds are commonly used. The first are the plastics, especially urethane or epoxy resins. These compounds, while effective, have low heat transfer characteristics, and are relatively expensive. The second, used in the electrical power and consumer appliance field, is a very inexpensive material, chosen for its low cost and good heat transfer and electrical properties. This potting compound comprises a mixture of asphalt, silica and, optionally, wax. The primary insulating and heat conducting compound is the silica, usually in the form of fine lean sand. Asphalt is used to provide an adhesive for the sand, and, as it melts at a relatively low temperature, to provide a liquid stage for pouring, and, upon cooling, a solid state for the potting compound. Wax modifies both the melt range of the compound and the adhesive characteristics of the asphalt, and is therefore present as a modifier for these characteristics.
Asphalt based potting compounds are applied by pouring the liquid, melted compound into a container containing the electrical circuit. The molten potting compound is usually contained in large, manually operated pouring pots, which hold the potting compound at the desired melt temperature for best pour characteristics. Such pots must usually be kept heated continuously, because the large thermal inertial of the potting compound requires too much heat to economically permit cool down and remelting for each day's work. As a result the heated pots require continuous power and monitoring for fire safety in a production environment.
Asphalt based potting compounds are designed to have a high heat conductance, for proper cooling of the encased electrical circuit. The potting compound usually has a relatively high specific heat. Electrical circuits, especially those containing heat sensitive components such as semiconductors or capacitors, have definite upper temperature limits. As an example, modern electrical fluorescent ballasts have components with an upper temperature range of 290 degrees F. There is therefore at all times a narrow temperature range at which the potting compound must be held to keep it liquid, and yet not overheat the electrical circuit when the compound is applied.
The current heat pot method of potting compound application therefore has several significant disadvantages. The pots must be kept continuously at full melt temperature, posing a continual explosion or fire hazard. The pots are expensive, it is estimated that a single production line requires 120,000 dollars solely for the heat pots. Finally, and of equal importance, the process requires manual judgement as to the amount of potting compound poured into each container which produces relatively uneven results and quality.